When I was ten I've spent half a year staying after school playing with water. I is wet, dirty, splashy and overall exiting. Bonus: explaining these experiments doesn't need much more than Archimedes force and p(rho)gh, so ten year old are capable of doing it. Try http://en.wikipedia.org/wiki/Cartesian_diver, different types of siphons ( including a so-called automatic one) and manometers, measuring the internal pressure in a balloon or even a bubble (why not?) well, the list is endless...